压铸镁合金AZ91D高周疲劳性能研究

研究了压铸镁合金AZ91D在应力比R=0．1条件下的高周疲劳性能。结果表明：AZ91D压铸镁合金的室温条件疲劳强度在应力比R=0．1时大约相当于其抗拉强度的44％；AZ91D合金内部的一些缺陷如夹杂等，容易引起应力集中，从而导致裂纹的萌生；AZ91D合金的疲劳断口可以观察到3个典型区域：疲劳源区、疲劳裂纹扩展区和瞬断区。疲劳裂纹扩展区的疲劳裂纹不明显，疲劳断口呈现出准解理断口的形貌。     

40CrNiMoA钢在水介质中的腐蚀疲劳行为

本文对40CrNiMoA钢在870℃淬火200℃回火和1200℃淬火200℃回火两种热处理状态下在蒸馏水中的应力腐蚀和腐蚀疲劳裂纹扩展过程进行了研究,考察了载荷波形、载荷频率、应力比、淬火温度等因素对腐蚀疲劳裂纹扩展特征和裂纹扩展速率da/dN的影响。依据本文的实验结果,对R.P.Wei和Landes提出的用来定量描述腐蚀疲劳裂纹扩展速率的叠加模型进行了计算,得出了该模型适用于40CrNiMoA钢——水介质条件的结论。并给出了可供实际使用的公式、数据和程序。     

2524铝合金不同应力比下的疲劳裂纹扩展行为

研究了不同应力比(R=0.1,0.2,0.3,0.5)对2524-T3铝合金的疲劳裂纹扩展速率的影响,并结合扫描电镜(SEM)下裂纹的断口形貌特征进行了分析。结果表明:应力比R对疲劳裂纹扩展寿命和速率存在明显影响,在恒定最大载荷条件下,应力比越小则裂纹扩展寿命越短;当应力强度因子范围△K相同时,应力比越大,裂纹扩展速率da/dN越快。利用Paris公式对数据进行回归分析,da/dN-△K曲线呈收敛趋势,进一步采用Elber模型进行拟合,从裂纹闭合的角度解释了应力比R对裂纹扩展速率的影响,结果表明该模型能够较好地描述2524-T3铝合金在不同应力比下的疲劳特性。疲劳断口呈现出早期裂纹扩展、稳定扩展和快速扩展3个阶段,不同应力比下,疲劳辉纹间距和二次裂纹数量存在明显差异。     

应力比对Mg-3Al-2Sc合金疲劳行为的影响

在不同应力比条件下对热挤压变形的Mg-3Al-2Sc合金进行了疲劳试验,分析了应力比对合金裂纹扩展行为和断裂机制的影响。结果表明:在应力幅Δσ=60 MPa时,随应力比的增加,Mg-3Al-2Sc合金疲劳寿命显著降低,裂纹扩展速率提高。在裂纹萌生区和扩展区疲劳断裂机制主要表现为解理断裂,而在瞬断区主要表现为准解理和沿晶断裂的复合断裂机制。     

一种Al-Cu-Mg-Zr合金的疲劳裂纹扩展行为研究

研究热处理状态、应力比和试样取向等对含Zr的Al-Cu-Mg系铝合金疲劳裂纹扩展行为的影响.结果表明,T3、T4两种热处理状态试样的疲劳裂纹扩展速率没有明显差别;L-T方向和T-L方向的试样也有大致相同的疲劳裂纹扩展速率;应力比R对疲劳裂纹扩展行为有明显影响,高应力比下的疲劳裂纹扩展速率明显快于低应力比条件下的扩展速率,并且高应力比下的疲劳裂纹扩展在较小的△K值下进入快速扩展阶段,并很快断裂.在较低的△K水平下应力比的影响与裂纹闭合效应有关.     

TB6钛合金疲劳及裂纹扩展性能研究

本文对TB6钛合金锻件弦向和径向两种取样方向分别进行了室温和200℃下旋转弯曲高周疲劳、轴向低周疲劳和疲劳裂纹扩展性能试验研究。试验结果表明,弦向（C）和径向（R）两种取样方向对该合金锻件的旋转弯曲高周疲劳、轴向低周疲劳性能和疲劳裂纹扩展性能没有影响;温度升高可加速该合金锻件的疲劳裂纹萌生,但在裂纹扩展阶段,该合金高温下的韧性优势与屈服强度降低的劣势平衡的结果使其在室温～200℃温度范围内的疲劳性能基本不受温度的影响;在10—20mm的厚度范围内,厚度对该合金的疲劳裂纹扩展性能没有影响;在3．5％NaCl盐雾环境中。腐蚀介质对TB6钛合金的疲劳裂纹扩展速率在初始阶段有迟滞作用,但在应力强度因子范围大于14MPa m后有加速作用。     

应力比对单晶高温合金高周疲劳行为的影响

为了研究应力比对DD6单晶高温合金高周疲劳行为的影响,在轴向疲劳试验机上分别测试了DD6合金1070℃条件下应力比为-1、-0.33、0.1、0.5、0.8的疲劳性能,采用扫描电镜(SEM)和能谱仪(EDS)观察分析了不同应力比合金的断口形貌和显微组织。结果表明：DD6合金不同应力比条件下的疲劳寿命均随应力幅值和平均应力的增加而降低;当应力幅值恒定时,DD6合金疲劳寿命随着应力比的增大而降低;当平均应力恒定时,应力比R＜0.5条件下的疲劳寿命随着应力比的增大而增加,同时,当平均应力增大到一定程度时,应力比对合金的疲劳寿命无明显影响。DD6合金低应力比条件下的疲劳断裂模式为类解理断裂,该条件下高温氧化有助于疲劳裂纹的萌生和扩展;高应力比条件下的疲劳断裂模式为韧窝断裂;中等应力比条件下疲劳断裂模式具有类解理断裂和韧窝断裂同时存在的特征。     

残余应力对U75V重轨钢疲劳裂纹扩展速率的影响

通过残余应力测试仪Stress3000测试了不同工艺处理U75V重轨钢表面残余应力,利用Sinco Tec疲劳试验机研究了残余应力对疲劳裂纹扩展速率的影响,并对疲劳断口形貌进行了观察。结果表明:不同工艺处理U75V重轨钢表面残余应力均为压应力,淬火态试样表面平均残余应力大于轧态试样;随退火温度升高和退火时间延长,试样表面残余应力下降明显。随表面残余压应力的增大,疲劳裂纹常数m值逐渐减小,即裂纹扩展速率随着表面残余压应力的增加而减小。断口形貌观察发现表面残余压应力较大时,断口疲劳辉纹更为细小,有利于降低表面疲劳裂纹扩展速率。     

2Cr12Ni4Mo3VNbN新型叶片钢高频淬火后的显微组织与疲劳性能

为提高2Cr12Ni4Mo3VNbN马氏体不锈钢叶片的疲劳性能,对调质处理后的叶片刃口区进行了高频淬火处理,研究了高频淬火前后2Cr12Ni4Mo3VNbN钢的显微组织、残余应力分布和疲劳性能。结果表明:2Cr12Ni4Mo3VNbN钢高频淬火后显微组织中的原奥氏体晶粒和马氏体板条束都得到细化,没有明显的析出物,而未高频淬火的马氏体有较多窄条状M3C析出。2Cr12Ni4Mo3VNbN叶片高频淬火区表层层深0.1 mm以内主要为压应力,应力值约在-450 MPa^-20 MPa之间,且随层深增加而减小。高频淬火态2Cr12Ni4Mo3VNbN钢的疲劳裂纹扩展门槛值为6.75 MPa·m^1/2,高于未高频淬火态下的4.73 MPa·m^1/2,并且高频淬火态2Cr12Ni4Mo3VNbN钢中的疲劳裂纹扩展速率低于未高频淬火态。这表明,高频淬火处理提高了2Cr12Ni4Mo3VNbN马氏体不锈钢的疲劳裂纹扩展抗力和疲劳裂纹扩展寿命。     

淬火速率对砂铸Al-7Si-0.4Mg合金拉伸性能及疲劳裂纹扩展行为的影响

研究了淬火速率对峰值时效状态下的砂铸Al-7Si-0.4Mg合金拉伸性能及疲劳裂纹扩展行为的影响.结果表明,随着淬火速率的提高,合金的抗拉强度、屈服强度以及α(Al)相的显微硬度均有提高,抗拉强度和屈服强度与淬火速率的对数值近似成线性关系.淬水速率对合金的延伸率影响较小,但对合金的室温疲劳裂纹扩展速率有明显影响,较高的淬火速率可以明显提高合金的疲劳裂纹扩展阻力.     

淬火残余应力对铝合金厚板裂纹应力强度因子及扩展趋势的影响

为探究淬火残余应力对铝合金厚板疲劳裂纹扩展的影响规律,建立7075铝合金厚板表面三维裂纹数值仿真模型。采用顺序热力耦合法求解淬火残余应力场,将残余应力场作为初始载荷条件求解裂纹应力强度因子,并与无残余应力场的应力强度因子值进行对比,研究两种条件下应力强度因子的分布规律和两者之间的异同;通过分析在初始淬火残余应力条件下不同半径裂纹受不同均匀拉应力荷载作用时的裂纹应力强度因子随裂纹位置角的演变曲线,探究淬火残余应力对裂纹扩展趋势的影响规律。结果表明,淬火残余应力的存在改变了铝合金厚板应力强度因子的分布规律和裂纹的扩展趋势,淬火残余应力使表层附近的裂纹扩展受到遏止,裂纹易于在厚度方向优先扩展。     

High cycle fatigue and fracture behavior of 2124-T851 aluminum alloy

The high cycle fatigue properties and fracture behavior of 2124-T851 aluminum alloy were investigated roundly, including the fatigue crack growth rate, fracture toughness and fatigue S--N curve. Furthermore, the fatigue crack growth rate was analyzed by fitting the curves. And the microstructure of the alloy was studied using by optical microscopy, transmission electron microscopy and X-ray diffractometry, scanning electron microscop1/2 The results show that the fatigue strength and the fracture toughness of 2124-T851 thick plate are 243 MPa and 29.6 MPa.m at room temperature and R=0. 1, respectively. At high cycle fatigue condition, the characteristics of fatigue facture were observed obviously. And the higher the stress amplitude, the wider the space between the fatigue striations, the faster the rate of fatigue crack developing and going into the intermittent fracture area and the greater the ratio between the intermittent fracture area and the whole fracture area.     

Effect of electromagnetic bulging on fatigue behavior of 5052 aluminum alloy

The effect of electromagnetic bulging on the fatigue behavior of the 5052 aluminum alloy was investigated through tensile–tensile fatigue testing. The intriguing finding is that the bulged specimens exhibited enhanced fatigue strength as depicted by maximum stress vs the number of cycles until failure (S–N) curves, by comparison with these original aluminum alloys. Although the fatigue process of the original and budged alloys follows the same mechanism with three distinct steps, namely, crack initiation at a corner of the tested samples, stable crack propagation with typical fatigue striations and finally catastrophic fracture with dimple fractographic features. The typical crack propagation rate vs stress intensity factor range (da/dN–ΔK) curves derived from the spacing of striations reveal a lower crack propagation rate in the bulged specimens. The enhancement of fatigue strength in electromagnetically bulged aluminum alloy is further rationalized in-depth on the basis of strain hardening and dislocation shielding effect.     

Effect of Temper Condition on the Corrosion and Fatigue Performance of AA2219 Aluminum Alloy

The effect of temper condition and corrosion on the fatigue behavior of alloy AA2219 has been investigated in different temper conditions (T87 and T851). Corrosion testing was performed by exposing the tensile specimens to 3.5% NaCl solution for different time periods, and the corrosion damage was quantified using a 3D profilometer. The exposure-tested specimens were subjected for fatigue testing at different stress levels, and the reduction in fatigue life was measured along with detailed fracture morphology variations. The results indicated that the alloy in both tempers suffers localized corrosion damage and the measured corrosion depth was 120 and 1200 µm, respectively, for T87 and T851 conditions. The loss in fatigue strength was found to be high for T851 (67%) when compared to that of T87 temper condition (58%) for a pre-corrosion time of 15 days. In both cases, fatigue crack initiation is associated with corrosion pits, which act as stress raisers. However, the crack propagation was predominantly transgranular for T87 and a mixed transgranular and intergranular fracture in the case of T851 temper condition. This was shown to be due to the heterogeneous microstructure due to the thermomechanical working and the delay in quench time imposed on the alloy forging in T851 temper condition. The findings in this paper present useful information for the selection of appropriate heat treatment condition to facilitate control of the corrosion behavior which is of great significance for their fatigue performance.     

Fatigue crack growth behavior of a solid solution-strengthened nickel-base superalloy (Incoloy 825)

Fatigue crack growth behavior of a solid solution-strengthened nickel-base superalloy (Incoloy 825)* was investigated. The investigation also examined the influence of heat treatment on resultant microstructures and the near-threshold fatigue crack growth behavior. In addition, the influence of load ratios(R), material strength, and grain size on fatigue threshold was studied. Compact tension specimens prepared from Incoloy 825 with transverse-longitudinal (TL) orientation in the as-received, as well as two different heat treated conditions, were used. The heat treatment studies revealed a peak hardness condition after solution treatment at 1200 °C for 1/2 hr, followed by aging at 600 °C for 434 hr. Among all the heat treated conditions, the fatigue threshold was the highest and the near-threshold crack growth rate was lowest in this peak aged condition. Fatigue threshold values were observed to decrease with an increase in load ratio, whereas an increased grain diameter resulted in a higher fatigue threshold. An earlier mathematical model was found applicable to characterize the relationship between load ratio and fatigue threshold. Preferential etching of grain boundary suggests formation of a thin film of carbide precipitation along the grain boundary region in the aged specimens. This carbide precipitation facilitated intergranular crack growth in these samples, resulting in higher roughness-induced crack closure. The highest fatigue threshold in the peak aged condition can be attributed to this large roughness-induced crack closure process. Incoloy 825 is a trademark for products of Huntington Alloys, Inc.     

High cyclic fatigue performance of Al–Cu–Mg–Ag alloy under T6 and T840 conditions

High cyclic fatigue (HCF) behavior of an AA2139 alloy belonging to Al–Cu–Mg–Ag system in T6 and T840 conditions was examined. The T840 treatment involving cold rolling with a 40% reduction prior to peak ageing provides an increase in tensile strength compared with the T6 condition. However, fatigue lifetime for two material conditions was nearly the same since there is weak effect of thermomechanical processing on micro-mechanisms of crack initiation and growth.     

含有第二相的高强铝合金疲劳模型

基于疲劳裂纹尖端的应力和应变以及高强铝合金中不同尺度第二相性态对其延性的影响，建立了高强铝合金中粗大第二相、中间尺度第二相以及细小时效强化相性态与其疲劳裂纹扩展速率之间的多元非线性关系模型。结果表明：对于2024铝合金的疲劳扩展速率，该模型的预测趋势与他人的实验研究结果吻合良好。同时借助于对该模型的理论分析，提出了在确保高强铝合金强度不降低的前提下降低其疲劳裂纹扩展速率的优化方案。     

S135钻杆钢预腐蚀后的弯曲疲劳性能

目的：考察有机盐钻井液对S135钻杆材料腐蚀及疲劳性能的影响。方法首先利用高温高压釜模拟有机盐钻井液井筒的工况环境,对疲劳试样进行预腐蚀,通过点蚀仪测定试样表面的腐蚀状况;然后利用旋转弯曲疲劳试验机在不同弯曲应力条件下对预腐蚀试样和未腐蚀试样的疲劳性能进行测试,算得不同存活率下的疲劳强度,并绘制不同存活率下的S-N曲线。用体视显微镜和扫描电镜观察预腐蚀试样和未腐蚀试样的疲劳断口形貌,进而得出S135钻杆材料表面腐蚀对其疲劳寿命的影响程度和影响机制。结果经过腐蚀的试样表面有较多腐蚀坑,腐蚀坑深度在0.4~0.7 mm之间。未腐蚀试样的疲劳强度为553 MPa,其疲劳断口只观察到单个疲劳裂纹源;腐蚀试样的疲劳强度为409 MPa,其疲劳断口观察到多个疲劳裂纹源。 S135钻杆材料腐蚀疲劳开裂敏感性指数为26%。结论经过高温高压有机盐钻井液环境腐蚀后,试样表面点蚀严重,腐蚀坑底部存在应力集中并导致裂纹源的形成,多个裂纹源的同时生长加快了裂纹的扩展,最终降低S135钻杆钢的疲劳强度。     

Fatigue properties of rolled AZ31B magnesium alloy plate

Fatigue strength, crack initiation and propagation behavior of rolled AZ31B magnesium alloy plate were investigated. Axial tension-compression fatigue tests were carried out with cylindrical smooth specimens. Two types of specimens were machined with the loading axis parallel (L-specimen) and perpendicular (T-specimen) to rolling direction. Monotonic compressive 0.2% proof stress, tensile strength and tensile elongation were similar for both specimens. On the other hand, monotonic tensile 0.2% proof stress of the L-specimen was slightly higher than that of the T-specimen. Moreover, monotonic compressive 0.2% proof stresses were lower than tensile ones for both specimens. The fatigue strengths of 107 cycles of the L- and T-specimens were 95 and 85 MPa, respectively. Compared with the monotonic compressive 0.2% proof stresses, the fatigue strengths were higher for both specimens. In other words, the fatigue crack did not initiate and propagate even though deformation twins were formed in compressive stress under the cyclic tension-compression loading. The fatigue crack initiated at early stage of the fatigue life in low cycle regime regardless of specimen direction. The crack growth rate of the L-specimen was slightly lower than of the T-specimen. Consequently, the fatigue lives of the L-specimen were longer than those of the T-specimen in low cycle regime.